The present invention relates to a copier capable of reproducing images on both faces of a sheet and/or reproducing composite images on the same surface of a sheet. More particularly, the present invention is concerned with an intermediate sheet feeder of such a copier which stacks sheets which have been moved past an image forming station and then feed them again to the same station.
A prior art intermediate sheet feeder of the type described which is installed in a copier with a double-face copying and/or a composite copying capability includes a single intermediate tray adapted to stack sheets thereon. The stack of sheets on the intermediate tray are sequentially re-fed by feed means. Sheets are loaded in a stack on the tray with their image-carrying surfaces sometimes faced upward and sometimes downward; usually face up in a both-face copy mode and face down in a composite copy mode. Where the prior art sheet feed means associated with the tray is constructed to sequentially feed the sheets beginning with the top one or with the bottom one, it feeds them beginning with the top one or with the bottom one without exception. The problem with such a construction is that toner on the image-carrying surface of each sheet is apt to adhere to that of the neighboring sheet to noticeably smear the surface of the latter sheet.
Meanwhile, various types of double-face copying systems are known in the art. Where a plurality of volumes of copies each of which is sorted in the same order as a plurality of orderly paged documents is to be produced, it is desirable that copies be provided in sorted bundles, or volumes, in order to save an extra step of sorting copies otherwise performed with a sorter. Especially, in a situation wherein bundles of copies are to be automatically bound one by one by a stapler, the presorted bundles of copies as stated above can be immediately bound as they come out of the copier. Both-face copying in such a mode may typically be implemented with any of two different methods as will be described.
In accordance with a first method, the first page of documents is reproduced on one surface of the first sheet, then the sheet is stacked on the intermediate sheet feeder, then the sheet is re-fed to reproduce the second document page on the other surface of the sheet, then the third and fourth document pages are sequentially reproduced on both faces of the second sheet. This procedure is repeated until all the documents have been reproduced, completing sorted bundles of copies on a volume basis. The drawback with this scheme is that the copying operation consumes a long period of time because after a certain document page has been copied on the first surface of a sheet the next document page cannot be copied until the same sheet returns by way of the intermediate sheet feeder. Especially, where an automatic sheet feeder is used to handle a number of documents within a short period of time, the above procedure increases the time necessary for copying and, thereby, causes half of the value of the automatic sheet feeder to be lost.
In relation to an automatic document feeder, a time-saving copying method is known as will be described. In accordance with this method, a plurality of documents are sequentially fed by an automatic document feeder to an imaging station inside a copier to reproduce documents every two pages on their first surfaces. The sheets are fed toward the intermediate sheet feeder to be sequentially stacked according to the order of reproduction. Then, the remaining documents are fed one by one by the automatic document feeder to the imaging station so as to be reproduced on the second surfaces of the sheets, which are sequentially fed again out of the intermediate sheet feeder from the top of the stack. The copies so provided with document images on their both faces are discharged to the outside of the copier. This allows a predetermined number of sheets each carrying images on its one surface to be stacked on the intermediate sheet feeder and then sequentially fed again to the imaging station, thereby reducing loss of time.
However, the critical problem with the every-two-pages scheme stated above is that when a sheet fed from the intermediate sheet feeder toward the imaging station has encountered a feed failure and has been discarded either alone or together with the others which lie in a transport path, it is difficult to supplement a substitute for the discarded copy. Specifically, to provide a substitute for the damaged copy, the same images as those which were formed on the first surface of the damaged copy are reproduced on the first surface of a fresh sheet and, then, the sheet is fed into the intermediate sheet feeder. In this case, if sheets which carry images on their first surfaces are present in the intermediate sheet feeder, then the incoming substitute will lie on top of the occupants. In order to sort the resulting bundle of copies, it is necessary for the substitute to be re-fed from the intermediate sheet feeder prior to the others, then provided with the same images as those which were expected to be reproduced on the second surface of the discarded sheet, and then moved out of the copier.
However, the intermediate sheet feeder cannot implement the re-feed of the substitute prior to the others which underlie the substitute, because it is constructed to re-feed a stack of sheets beginning with the bottom sheet. The only expedient heretofore available to cope with such a situation has been discarding the whole stack of copies remaining on the intermediate sheet feeder and then resuming the copying operation, or completing reproduction on all the remaining copies, then producing a substitute for the discarded copy, and then inserting the substitute into the bundle of copies outside of the copier. The former scheme, however, not only leads to the waste of sheets but also invites substantial loss of time due to the extra copying operation. The latter scheme, on the other hand, results in considerably troublesome manipulation especially when the number of copies is great or when a stapler is connected to the copier.